Abstract: To meet the test requirement of the satellites belonging to the SAST5000 satellite platform and the SAR series, a large space environmental test facility, named KM5B, is built. The KM5B facility, which includes a main erect chamber and an auxilliary horizontal chamber, can be expanded according to the requirement. This paper briefly introduces the technical specifications, the structure configuration, the key technologies of the KM5B facility, which is the second largest space environmental test facility in use in China.